An anti-theft system for a motor vehicle is known from DE 44 30 360 C1, where a stationary transmitter in a lock comprises a resonant circuit which is inductively linked to an resonant circuit of a portable transponder in a key. In the transmitter an oscillation is triggered by an excitation variable of which the energy is transmitted to a transponder which, for its part, transmits coded data back to the transmitter.
Below a specific power threshold the energy transmitted to the transponder device is too small for a transmission of code information by the transponder device. To guarantee a secure exchange of energy and data between the base station and the transponder unit the transmit power of the base station must thus be set to a specific minimum value.
DE 196 02 316 C1 discloses a device for transmission of data or energy which can be used especially for an anti-theft system for a motor vehicle, in which coded information is sent from a key to a lock and in the opposite direction. The document proposes the use of a stationary transceiver with a first resonant circuit which is excited to an oscillation by an excitation variable with an excitation frequency. During the manufacturing of the transceiver, deviations from required values in components of the resonant circuit can occur as a result of component tolerances. After the transceiver has been manufactured it must therefore be first established whether its resonant frequency deviates by more than a specified amount from the excitation frequency. To this end the amplitude of the current through the antenna or the voltage at a capacitor of the resonant circuit is measured. If the current or the voltage deviates from a required value, a correction value is created. In accordance with an exemplary embodiment described in DE 196 02 316 C1, after the transceiver has been manufactured, it must first be established whether a resonance frequency deviates from the excitation frequency by more than a specified amount. The antenna resonant circuit can be changed in two different ways independently of the correction value so that energy and data are transmitted in the most effective way possible. On the one hand the resonant frequency of the antenna resonant circuit can be approximated to the excitation frequency and on the other hand the current through the antenna, i.e. the excitation current, can be increased. A measuring device with a microprocessor can be used for measuring current and voltage, which records the current via an A/D converter and undertakes to connect a capacitor and/or an inductor or to control the excitation current.